Embodiments of the present invention relate to printers and, more particularly to a printhead for a printer.
Printers typically have a printhead mounted on a carriage that scans back and forth across the width of a sheet of paper, as the paper is fed through the printer. Fluid from a fluid reservoir, either on-board the carriage or external to the carriage, is fed to fluid ejection chambers on the printhead. Each fluid ejection chamber contains a fluid ejection element, such as a heater resistor or a piezoelectric element, which is independently addressable. Energizing a fluid ejection element causes a droplet of fluid to be ejected through a nozzle to create.a small dot on the paper. The pattern of dots created forms an image or text.
Hewlett-Packard is developing printheads that are formed using integrated circuit techniques. A thin film membrane, composed of various thin film layers, including a resistive layer, is formed on a top surface of a silicon substrate, and an orifice layer is formed on top of the thin film membrane. The various thin film layers of the thin film membrane are etched to provide conductive leads to fluid ejection elements, which may be heater resistor or piezoelectric elements. Fluid feed holes are also formed in the thin film layers. The fluid feed holes control the flow of fluid to the fluid ejection elements. The fluid flows from the fluid reservoir, across a bottom surface of the silicon substrate, into a trench formed in the silicon substrate, through the fluid feed holes, and into fluid ejection chambers where the fluid ejection elements are located.
The trench is etched in the bottom surface of the silicon substrate so that fluid can flow into the trench and into each fluid ejection chamber through the fluid feed holes formed in the thin film membrane. The trench completely etches away portions of the substrate near the fluid feed holes, so that the thin film membrane forms a shelf in the vicinity of the fluid feed holes.
One problem faced during development of these printheads is that the conductive leads in the thin film membrane extend over the trench and can develop cracks when the printhead is flexed or otherwise subjected to stress. Stresses can occur during assembly and operation of the printhead. When cracks propagate and intersect active resistor lines, they can cause a functional failure in the printhead. A crack that initially incapacitates a single resistor allows fluid to access the aluminum conductor. Aluminum corrodes quickly in fluid, particularly when supplied with an electrical potential to drive galvanic reactions. As a result, the problem that started with a single resistor can quickly spread to multiple nozzles or the entire printhead, as the corrosive fluid attacks the power bus. Thus, there is a need for an improved printhead that maintains its reliability throughout assembly and operation.
Described herein is a printhead having a printhead substrate and a thin film membrane. The printhead substrate has at least one opening formed therein for providing a fluid path through the substrate. The thin film membrane is formed on a second surface of the substrate and extends over the opening in the substrate. The thin film membrane includes a plurality of fluid feed holes. Each fluid feed hole is located over the opening in the substrate. The thin film membrane further includes a plurality of fluid ejection elements and a plurality of conductive leads to the fluid ejection elements. All portions of the fluid ejection elements and conductive leads overlie the substrate.